Systems and methods for operating passive nitrogen oxide adsorbers in exhaust aftertreatment systems

ABSTRACT

A system includes a catalyst for receiving and treating exhaust gas generated by an engine, a passive NOx adsorber (PNA) positioned upstream of the catalyst, a bypass valve positioned upstream of the catalyst and the PNA, and a controller. The controller is configured to, determine that the catalyst is operating under cold start conditions, control the bypass valve to direct exhaust gas to the PNA, determine that the catalyst is no longer operating under cold start conditions and continue to control the bypass valve to direct exhaust gas to the PNA for a predetermined duration, and after the elapse of the predetermined duration, control the bypass valve to direct exhaust gas to the catalyst bypassing the PNA. The controller is also configured to detect a high transient torque demand while the exhaust gas is provided to the PNA, and split the torque demand between the engine and an electric motor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to, and the benefit of the filing dateof U.S. Provisional Patent Application No. 62/938,499, filed Nov. 21,2019, the entire disclosure of which is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present disclosure relates to internal combustion engine basedsystems, and in particular to hybrid systems.

BACKGROUND

For internal combustion engines such as, for example, diesel engines,nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduceNOx emissions, a selective catalytic reduction (SCR) process may beimplemented to convert the NOx compounds into more neutral compounds,such as diatomic nitrogen, water, or carbon dioxide, with the aid of acatalyst and a reductant. The catalyst may be included in a catalystchamber of an exhaust system, such as, for example, that of a vehicle orpower generation unit. A reductant, such as anhydrous ammonia, aqueousammonia, diesel exhaust fluid (DEF), or aqueous urea, is typicallyintroduced into the exhaust gas flow prior to the SCR catalyst. Tointroduce the reductant into the exhaust gas flow for the SCR process,an SCR system may dose or otherwise introduce the reductant through adosing module that vaporizes or sprays the reductant into an exhaustpipe of the exhaust system up-stream of the catalyst chamber. The SCRsystem may include one or more sensors to monitor conditions within theexhaust system.

SUMMARY

In one aspect, a system includes a catalyst for receiving and treatingexhaust gas generated by an engine, a passive NOx adsorber (PNA)positioned upstream of the SCR catalyst and fluidly coupled with thecatalyst, a bypass valve positioned upstream of the PNA, and acontroller. The controller is configured to, determine that aneffectiveness of the catalyst in reducing NOx is below a thresholdlevel, in response to determining that the effectiveness of the catalystin reducing NOx is below the threshold value, control the bypass valveto direct exhaust gas to the PNA. The controller is further configuredto, subsequent to controlling the bypass valve to direct exhaust gas tothe PNA, determine that the effectiveness of the catalyst is no longerbelow the threshold level. The controller is further configured to, inresponse to determining that the effectiveness of the catalyst is nolonger below the threshold value, continue to control the bypass valveto direct exhaust gas to the PNA, and after PNA regeneration conditionsare met, control the bypass valve to direct exhaust gas to the catalystbypassing the PNA.

In another aspect, a system includes a catalyst for receiving andtreating exhaust gas generated by an engine, a PNA positioned upstreamof the catalyst and fluidly coupled with the catalyst, a bypass valvepositioned upstream of the catalyst and the PNA, and a controller. Thecontroller is configured to, while controlling the bypass valve todirect exhaust gas to the PNA, detect a torque demand that is greaterthan a threshold value. The controller is further configured to,responsive to detecting that the torque demand is greater than thethreshold value, engage a motor, coupled with a battery system, with adrive shaft of the system to meet at least a portion of the torquedemand. The controller is further configured to, in response to theengagement of the motor with the drive shaft not meeting all of thetorque demand, engage the engine with the drive shaft to meet aremainder of the torque demand.

In yet another aspect, a method for operating passive nitrogen oxideabsorbers (PNA) in an exhaust aftertreatment system includes determiningthat an effectiveness of a catalyst in reducing NOx is below a thresholdlevel, the catalyst for receiving and treating exhaust gas generated byan engine. In response to determining that the effectiveness of thecatalyst in reducing NOx is below the threshold level, the methodfurther includes controlling a bypass valve to direct exhaust gas to thePNA, the PNA positioned upstream of the catalyst and fluidly coupledwith the catalyst and the bypass valve positioned upstream of thecatalyst and the PNA. Subsequent to controlling the bypass valve todirect exhaust gas to the PNA, the method further includes determiningthat the effectiveness of the catalyst is no longer below the thresholdlevel. In response to determining that the effectiveness of the catalystis no longer below the threshold level, the method further includescontinuing to control the bypass valve to direct exhaust gas to the PNA,and after PNA regeneration conditions are met, controlling the bypassvalve to direct exhaust gas to the catalyst bypassing the PNA

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of thesubject matter described herein. The drawings are not necessarily toscale; in some instances, various aspects of the subject matterdisclosed herein may be shown exaggerated or enlarged in the drawings tofacilitate an understanding of different features. In the drawings, likereference characters generally refer to like features (e.g.,functionally similar and/or structurally similar elements).

FIG. 1 shows a block diagram of an example vehicle system, according toan embodiment of the present disclosure.

FIG. 2 shows a block diagram showing an internal combustion engine andexhaust aftertreatment system, according to an embodiment of the presentdisclosure.

FIG. 3 shows a flow diagram of an example process for controlling avehicle system shown in FIG. 1.

FIG. 4 shows a block diagram of the example internal combustion engineand exhaust aftertreatment system shown in FIG. 2 including a portion ofa path of the exhaust gas under cold start conditions.

FIG. 5 shows a block diagram of the example internal combustion engineand exhaust aftertreatment system shown in FIG. 2 including a portion ofa path of the exhaust gas when not under cold start conditions.

FIG. 6 shows plots of torque demand and lambda for an engine ininstances where only the engine is utilized to meet high transienttorque demand, according to various embodiments of the presentdisclosure.

FIG. 7 shows a flow diagram of an example process for operation of thevehicle system under high transient torque demand.

FIG. 8 shows example torque and lambda plots for the vehicle whenoperating in a power split mode to reduce the risk of rich air-fuelmixture operation.

The features and advantages of the inventive concepts disclosed hereinwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, methods, apparatuses, and systems forexhaust aftertreatment. It should be appreciated that various conceptsintroduced above and discussed in greater detail below may beimplemented in any of numerous ways, as the disclosed concepts are notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

Internal combustion engines (e.g., diesel or gasoline internalcombustion engines, etc.) produce exhaust gases that are often cleanedwithin an aftertreatment system. The aftertreatment system can include adecomposition chamber that converts a reductant, such as urea or DEF,into ammonia. The ammonia is mixed with the exhaust and provided to anSCR catalyst. The SCR catalyst is configured to assist the reduction ofNOx emissions in the exhaust gas by accelerating a NOx reduction processbetween the ammonia and the NOx of the exhaust gas into diatomicnitrogen, water, and/or carbon dioxide.

The SCR catalyst operation can be affected by several factors. Forexample, the effectiveness of the SCR catalyst to reduce the NOx in theexhaust gas can be affected by the operating temperature. If thetemperature of the SCR catalyst is below a threshold value, theeffectiveness of the SCR catalyst in reducing NOx may be reduced below athreshold level, thereby increasing the risk of high NOx emissions intothe environment. The SCR catalyst temperature can be below the thresholdtemperature under several conditions, such as, for example, during andimmediately after engine startup, during cold environmental conditions,etc. In hybrid systems that either use internal combustion engines forcharging batteries or to provide power in conjunction with one or moreelectric motors, the internal combustion engine may start and stop atvariable times, increasing the risk of low SCR catalyst temperatures. Asa result, when the engine is started, the low temperature of the SCRcatalyst can result in high NOx emission levels in the exhaust. Whilethe SCR catalyst temperature could progressively increase once theengine is running after startup, until that time, the exhaust gas caninclude an undesirable amount of NOx. The effectiveness of the SCRcatalyst can also be affected by faults in the SCR catalyst, such as,for example, a lack of reductant or a clogging of the SCR catalyst.

In some instances, PNAs can positioned upstream of the SCR catalyst toadsorb NOx when the effectiveness of the SCR catalyst is below athreshold level. For example, the exhaust gas can be passed through thePNA prior to passing the exhaust gas through the SCR catalyst until theSCR catalyst temperature is at a desired threshold value. Thereafter,the PNA can be bypassed and the exhaust gas can be directed to the SCRcatalyst without passing through the PNA.

The PNA includes active adsorption sites, to which NOx is adsorbed. Forexample, the PNA can include metal ions, such as, palladium, platinum,or silver ions, to which the NOx is adsorbed. Engaging the PNA in theexhaust gas path to adsorb NOx can cause the number of sites for NOxadsorption to reduce over time. Thereafter, the PNA is disengaged whenthe effectiveness of the SCR catalyst rises above the threshold level.However, when the PNA is reengaged, say for example, when the engine isrestarted on a subsequent occasion, the reduction in the number of sitesfor NOx adsorption reduces the effectiveness of the PNA over time.

In some instances, the effectiveness of the PNA to adsorb NOx maydeteriorate under exposure to exhaust gas generated under rich air-fuelmixture conditions. The exposure to rich air-fuel mixture may furtherdeteriorate the ability of the PNA to adsorb NOx at the next cold start.

The systems and methods discussed herein provide solutions to theproblems of reduced number of active sites for NOx adsorption and fordeterioration of the effectiveness of the PNA in adsorbing NOx due toexposure to exhaust gas generated under rich air-fuel mixtureconditions. In some embodiments, the PNA can be regenerated using theexhaust gas after the SCR catalyst has reached a temperature at whichthe SCR can effectively reduce NOx. In particular, the exhaust gas at orabove a particular temperature can be passed through the PNA to desorbthe already adsorbed NOx from the active sites, thereby re-exposingthose active sites to adsorb NOx when the PNA is reengaged on asubsequent occasion. In some embodiments, the combined operation of theengine and a motor generator of the hybrid system can be controlled toreduce the risk of rich air-fuel mixture conditions during hightransient torque demand, thereby reducing the risk of deterioration ofthe PNA. The discussion below provides solutions to problems discussedabove in relation to PNAs used in aftertreatment systems.

FIG. 1 shows a block diagram of a portion of an example hybrid vehiclesystem 100. The system 100 includes an internal combustion engine 102,an electrical motor 104, a generator 106, a battery system 108, anexhaust aftertreatment system 110, a drive shaft 112 coupled to drivewheels 114, an axle 120, and a drivetrain 116. The engine 102 can be anycombustion engine that converts energy generated by combustion of afuel, such as for example, gasoline, diesel, ethanol, etc., intomechanical energy. For example, the engine 102 can be a heavy-dutyinternal combustion diesel engine. The motor 104 can be, for example, aseries electrical motor. The generator 106 converts mechanical energyinto electrical energy, which can be utilized to recharge the batterysystem 108 or to provide electrical power to the motor 104. The batterysystem 108 can include rechargeable batteries or capacitive chargestorage to store electrical power. The battery system 108 can includeadditional circuitry that can convert the electrical energy provided bythe generator 106 or the motor 104 into suitable voltage and current tocharge the batteries or capacitive storage. For example, the batterysystem 108 can include an inverter that converts alternating voltage andcurrent generated by the generator 106 into direct voltage and currentof appropriate magnitudes to recharge the batteries or the capacitivecharge storage.

The engine 102 can be mechanically coupled with the generator 106 andthe electrical motor 104 via the drivetrain 116. For example, the crankshaft or an output shaft of each of the engine 102, the motor 104 andthe generator 106 can be coupled with the drivetrain 116. In someexamples, the drivetrain 116 can be a series drive train, in which thedrive shaft 112 is driven only by the power delivered from the motor104. In such instances, the engine 102 can be coupled with the generator106 (which may also operate as a starter motor) to generate electricalenergy to charge the battery system 108 and/or to provide electricalenergy to drive the motor 104. In some examples, the drivetrain 116 canbe a parallel drivetrain, in which mechanical power generated by boththe engine 102 and the motor 104 can be selectively provided to thedrive shaft 112. The mechanical power generated by the engine 102 alone,the mechanical power generated by the motor 104 alone, or thecombination of the mechanical power generated by the engine 102 and themotor 104 can be selectively provided to the drive shaft 112. In somesuch examples, the motor 104 can also be used as a generator to chargethe battery system 108 by converting mechanical power received from theengine 102 or from the drive shaft 112 during regenerative braking. Thedrivetrain 116 can include a transmission and one or more clutchmechanism to allow engagement and disengagement of the engine 102, motor104 and the generator 106 from each other and the drive shaft 112 (andaxle 120). The drivetrain 116 can be controlled by the controller 118.

The exhaust aftertreatment system 110 receives exhaust gas from theexhaust manifold of the engine 102 and processes the exhaust gas toremove particulate matter and to reduce the amount of NOx emissions intothe environment. The system 100 also includes a controller 118 thatcontrols the operation of at least the above mentioned components of thesystem 100. The controller 118 can include one or more of a programmablemicrocontroller or a microprocessor, a logic circuit, a digital/analogcircuit, a programmable logic circuit, a field programmable logic gatearray, a memory, etc. The controller 118 receives inputs from one ormore components in the system 100 and provides control signals toactuate one or more actuators or circuits within the system 100. Thecontroller 118 can be communicably coupled to a memory (volatile ornon-volatile), which can store data and instructions that can beexecuted by the controller 118. In some instances, the data andinstructions can be stored in one or more non-volatile computer readablestorage mediums, such as, for example, flash drives, compact discs,read-only-memories (ROMs), tape drives, cloud storage, etc.

FIG. 2 shows a block diagram of an example exhaust aftertreatment system200 downstream of the engine 102. The exhaust aftertreatment system 200can be used to implement the exhaust aftertreatment system 110 discussedabove in relation to FIG. 1. The exhaust aftertreatment system 200includes at least a PNA 202, an SCR catalyst 204 and a bypass valve 206.Other components, such as a diesel oxidation catalyst (DOC), aparticulate filter (specifically a diesel particulate filter (DPF)), anammonia slip catalyst (ASC), DEF dosers, mixers, sensors (temperatureand NOx) and/or others could be also included at various locations.Exhaust gases from the engine's 102 exhaust manifold are provided to aninput conduit 208 of the aftertreatment system 200. The bypass valve 206is positioned downstream of the exhaust manifold of the engine 102 andthe input conduit 208 of the aftertreatment system 200. The bypass valve206 is a multi-way valve, such as, for example a three-way valve, havingat least one input port and at least two output ports. For example, oneinput port of the bypass valve 206 is coupled with the input conduit208, one output port of the bypass valve 206 is coupled with a bypassconduit 210 and another output port of the bypass valve 206 is coupledwith a PNA conduit 212. The bypass valve 206 can be controlled toselectively direct exhaust gases received at its input port to thebypass conduit 210 or to the PNA conduit 212. For example, in a firstposition, the bypass valve 206 directs the exhaust gas from the engine102 to the PNA 202 via the PNA conduit 212, and in a second position,the bypass valve 206 directs the exhaust gas to bypass the PNA 202 viathe bypass conduit 210. The bypass valve 206 can include positions inaddition to the first and the second positions in which the exhaust gasreceived at its input port is proportionally output between the PNAconduit 212 and the bypass conduit 210. The bypass valve 206 can beelectronically controlled by a bypass valve signal received from acontroller, such as the controller 118 shown in FIG. 1.

The PNA 202 is positioned downstream of the bypass valve 206 and the PNAconduit 212 and upstream of a SCR catalyst conduit 214 fluidly couplingthe PNA 202 with the SCR catalyst 204. The PNA 202 can include NOxadsorbing elements, such as, for example, palladium, platinum, sliver,zeolite, Al₂O₃, CeO₂-containing materials, etc., that can adsorb NOx inthe exhaust gas. The PNA 202 can receive the exhaust gas produced by theengine 102 and directed to the PNA conduit 212 by the bypass valve 206,adsorb the NOx in the exhaust gas.

The SCR catalyst 204 is configured to assist in the reduction of NOxemissions in the exhaust gas by accelerating a NOx reduction processbetween a reductant, such as ammonia or urea, and the NOx in the exhaustgas into diatomic nitrogen, water, and/or carbon dioxide. The SCRcatalyst 204 is coupled downstream of the engine 102 and the PNA 202.The SCR catalyst 204 can receive exhaust gas from the PNA 202 or fromupstream of the PNA 202 via the bypass conduit 210. The output of theSCR catalyst 204 is provided to an output conduit 216, which can connectto other components of the aftertreatment system such as, for example,the ASC, a muffler or a tail pipe.

An SCR catalyst temperature sensor 218 senses the temperature of the SCRcatalyst 204. As mentioned above, the effectiveness of the SCR catalyst204 can be reduced at low temperatures. The SCR catalyst temperaturesensor 218 measures the operating temperature of the SCR catalyst 204and provides the temperature reading to a controller, which can use themeasured temperature to identify a cold start. For example, when theengine 102 is started after a long interval, the temperature of the SCRcatalyst 204 can be below a threshold value. The controller can comparethe measured temperature to a threshold value, and if the temperature isbelow the threshold value, the controller can determine a cold start. Anengine coolant temperature sensor 220 senses the temperature of thecoolant of the engine 102 and provides the sensed temperature to thecontroller. The controller can also consider the engine coolanttemperature to determine that a cold start is occurring. For example,the controller can determine a cold start if the engine coolanttemperature provided by the engine coolant temperature sensor 220 isbelow a threshold value. In some examples, the controller can use thetemperature reading provided by the SCR catalyst temperature sensor 218and the temperature reading provided by the engine coolant temperaturesensor 220 separately or in combination to determine a cold start. Thethreshold temperatures to which the controller can compare the receivedtemperature readings can be a function of the particular SCR catalystused, or the aging state of the SCR catalyst, as different SCR catalystsor aging states can have different effectiveness profiles with respectto temperature.

In some embodiments, a NOx sensor can sense the NOx at the output of theSCR catalyst 204 (such as, for example, at the output conduit 216) todirectly measure the effectiveness of the SCR catalyst 204. The NOxsensor can provide a measure of concentration (e.g., inparts-per-million (ppm)) of NOx in the exhaust gas at the output of theSCR catalyst 204. The controller can compare the measurement receivedfrom the NOx sensor to a threshold value, and if below the thresholdvalue, determine that the effectiveness of the SCR catalyst 204 toreduce NOx is below a threshold level. The controller can consider acombination of the temperature sensors 218 and 220, and the NOx sensorto determine the effectiveness of the SCR catalyst 204. In someembodiments, additional sensors such as a reductant sensor can determinewhether the reductant supplied to the SCR catalyst 204 has been used up,thereby reducing the effectiveness of the SCR catalyst 204 in reducingNOx. That is, the controller can compare the measurement received fromthe reductant sensor to a threshold value, and if below the thresholdvalue, determine that the effectiveness of the SCR catalyst 204 toreduce NOx is below a threshold level.

FIG. 3 shows a flow diagram of an example process 300 for controlling avehicle system. The controller 118 shown in FIG. 1, for example, canexecute the process 300 to control the exhaust aftertreatment system 110shown in FIG. 1 (or the exhaust aftertreatment system 200 shown in FIG.2), and various components of the vehicle system 100. The controller 118can execute the process 300 to determine whether the effectiveness ofthe SCR catalyst 204 is below a threshold level and control the exhaustaftertreatment system 110 based on the determination. For example, thecontroller 118 can determine whether the SCR catalyst 204 is operatingunder cold start conditions, during which the effectiveness of the SCRcatalyst 204 to reduce NOx is diminished. While the process 300 focuseson the cold start conditions of the SCR catalyst 204, the process 300can be readily adapted to determining the effectiveness of the SCRcatalyst 204 based on other measurements, such as, for example, the NOxsensors, or the reductant sensor. The process 300 includes receivingtemperature measurements from temperature sensors (302). The controller118 receives temperature measurements from one or more temperaturesensors of the vehicle system 100. For example, the controller 118 canreceive temperature measurements from the SCR catalyst temperaturesensor 218 and/or the engine coolant temperature sensor 220.

The process 300 further includes determining whether the measuredtemperature is less than a threshold value (304). For example, thecontroller 118 compares the temperature measurement (T) received fromthe SCR catalyst temperature sensor 218 to a threshold value (Tth). Ifthe measured temperature is less than the threshold value, thecontroller 118 can identify a cold start condition. When the engine 102is started, the exhaust gas from the engine is processed by the SCRcatalyst 204 to reduce the NOx in the exhaust gas. Nevertheless, theeffectiveness of the SCR catalyst 204 to reduce the amount of NOx in theexhaust gas can be diminished if the SCR catalyst 204 is operating atlow temperatures. Cold start conditions can represent conditions thatcan diminish the effectiveness of the SCR catalyst 204 to reduce NOx inthe exhaust gas below a desired level at and after engine startup. Acold start condition can occur, for example, when the temperature of theSCR catalyst 204 is below a certain value. This temperature value can bespecific to the type of SCR catalyst 204. As an example, the temperaturevalue can be about 200 degrees Celsius. That is, if the temperature ofthe SCR catalyst 204 is below 200 degrees Celsius, the effectiveness ofthe SCR catalyst can diminish below the desired level. The controller118 can detect a cold start condition by determining that thetemperature measurement received from the SCR catalyst temperaturesensor 218 is below the predetermined threshold value.

The controller 118 also may consider temperature measurements fromalternative or additional locations to determine whether the vehiclesystem 100 or the SCR catalyst 204 is in a cold start condition. Forexample, the controller 118 may take into consideration the engine 102coolant temperature received from the engine coolant temperature sensor220. Considering additional temperature measurements can reduce the riskof incorrect detection of a cold start. The controller 118 can comparethe temperature measurements received from the more than one temperaturesensors to their respective threshold values, and based on the outcomedetermine whether a cold start condition has occurred.

The controller 118 can determine the occurrence of cold start conditionsimmediately after the engine has been started. The controller 118 mayalso determine the occurrence of the cold start conditions prior to theengine being started, such as, for example, when the starter motor isactivated or when electric power to the vehicle is turned on.Determining a cold start condition prior to starting the engine canallow the controller 118 to configure the system even before the startof the engine to compensate for the lack of NOx capture by the SCRcatalyst 204.

Upon determining that the vehicle system 100 is operating under coldstart conditions, the controller 118 controls the bypass valve 206 suchthat the exhaust gas received from the engine 102 is directed to the PNA202 (306). FIG. 4 shows a block diagram of the example exhaustaftertreatment system 200 shown in FIG. 2 including a portion of a pathof the exhaust gas under cold start conditions. In particular, FIG. 4shows a path 402 of the exhaust gas from the engine 102, through thebypass valve 206 and the PNA 202 and to the SCR catalyst 204. Thecontroller 118, upon determining a cold start condition, controls thebypass valve 206 such that the exhaust gas received by the bypass valve206 via the input conduit 208 is directed to the PNA conduit 212. Insome implementations, the controller 118 can control the bypass valve206 such that no exhaust gas is directed to the bypass conduit 210. Inother implementations, the controller 118 may control the bypass valve206 such that most—but not all—of the exhaust gas (e.g., at least 90% ofthe exhaust gas) received at the input conduit 208 is directed to thePNA conduit 212.

The PNA 202 compensates for the loss in effectiveness of the SCRcatalyst 204 in reducing NOx in the exhaust gas under cold startconditions. The PNA 202 adsorbs NOx that would otherwise have beenreleased into the environment by the SCR catalyst 204 under cold startconditions. The controller 118 monitors the temperature of the SCRcatalyst to determine whether the temperature has increased above thethreshold value (308). As long as the temperature is below the thresholdvalue, the controller 118 identifies that the vehicle system 100 isstill under cold start conditions. Therefore, the controller 118continues to control the bypass valve 206 to direct the exhaust gasthrough the PNA 202 before being provided to the SCR catalyst 204. Thethreshold value with which the controller 118 compares the temperatureof the SCR catalyst 204 can be the same as the threshold value used instep 304. With time, the temperature of the SCR catalyst 204 can risedue to exposure to high temperature exhaust gas.

If the controller 118 determines that the temperature of the SCRcatalyst 204 is above the threshold value, the controller 118 identifiesthat the cold start condition has ended. That is, the temperature of theSCR catalyst 204 has reached a level where the SCR catalyst 204 caneffectively reduce the NOx in the exhaust gas. The fact that the SCRcatalyst 204 is now operating effectively may justify the removing thePNA 202 from the exhaust gas path. However, the controller 118 cancontinue to direct the exhaust gas through the PNA 202 to regenerate thePNA 202 (310) by affecting desorption of NOx from the PNA 202. DuringNOx release regeneration process, the PNA 202 is exposed to hightemperatures for a prescribed duration to release or desorb the NOxadsorbed therein, thereby increasing the density of active adsorptionsites. However, unlike traditional approaches, which utilize separateheaters and pumps to expose the PNA 202 to higher temperatures, theapproach discussed herein instead utilizes the exhaust gas itself toprovide heat to regenerate the PNA 202. For example, the controller 118,after detecting that the vehicle system 100 is no longer in a cold startcondition, continues to maintain the bypass valve 206 in a position thatdirects the exhaust gas received from the engine 102 towards the PNA202. The high temperature exhaust gas can provide the heat energy toregenerate the PNA 202 by releasing the stored NOx.

The controller 118 maintains the position of the bypass valve 206,thereby continuing to provide exhaust gas to the PNA 202, until a NOxrelease regeneration condition is satisfied (312). The regenerationcondition can be satisfied when a duration for which the PNA 202 isexposed to the high temperature exhaust gas exceeds a predeterminedthreshold value. For example, the controller 118 can start a timer afterdetermining an end of the cold start condition, and consider theregeneration condition satisfied when the timer reaches a thresholdvalue. The threshold value can depend upon the type of PNA utilized, andcan vary with various implementations of the PNA. In one example, thethreshold value can be about one to five minutes. That is, thecontroller can expose the PNA 202 to high temperature exhaust gas forabout one to five minutes after the end of the cold start condition.

In some instances, the regeneration condition can be satisfied when acombination of exhaust gas temperature and duration are satisfied. ThePNA 202 regeneration can be a function of both the temperature of theexhaust gas and the duration for which the PNA 202 is exposed to theexhaust gas. If the temperature is increased, the duration of theregeneration process can be reduced, and vice versa. The controller 118monitors the temperature of the exhaust gas by receiving measurementsfrom one or more temperature sensors positioned along the path of theexhaust gas, such as for example, positioned upstream of the PNA 202, oreven from the SCR catalyst temperature sensor 218. The controller 118then computes the duration of the regeneration process based on themeasured temperature. For example, the controller 118 determines theduration based on a formula, or a look up table, that can provide aduration value based on the measured temperature value.

In some instances, the controller 118 operates the engine 102 such thatthe temperature of the exhaust gas is increased during the regenerationprocess. In one example, the controller 118 increases the load on theengine 102 by engaging the generator (generator 106, FIG. 1, or motor104 if the motor is utilized as a generator) with the engine 102. Thegenerator 106 can convert the mechanical power provided by the engine102 into electrical energy to charge the battery system 108. Theincreased load on the engine 102 causes an increase in the temperatureof the exhaust gas, which can improve the rate of regeneration processof the PNA 202. Further, the controller 118 can simultaneouslyregenerate the PNA 202 and charge the battery system 108. Even thoughthe increased load on the engine 102 may result in higher fuelconsumption, at least a portion of the mechanical energy generated bythe engine 102 is converted to electrical energy and stored in thebattery system 108. This stored electrical energy can in turn beutilized to power the motor 104, thereby improving the overall fuelefficiency of the system. By regenerating the PNA 202 immediately afterthe cold start condition has ended improves the effectiveness of the PNA202 to adsorb NOx in a subsequent cold start condition.

Once the controller 118 determines that the regeneration condition issatisfied, the controller 118 controls the bypass valve 206 to directthe exhaust gas towards the bypass conduit 210, thereby bypassing thePNA 202 (314). FIG. 5 shows a block diagram of the example exhaustaftertreatment system 200 shown in FIG. 2 including a portion of a path502 of the exhaust gas when not under cold start conditions. Thecontroller 118 controls the bypass valve 206 such that the exhaust gasreceived via the input conduit 208 is directed to the bypass conduit 210and towards the SCR catalyst 204, thereby bypassing the PNA 202. As theSCR catalyst 204 is at a temperature where it can effectively reduce theamount of NOx in the exhaust gas, the PNA 202 is not needed. Thus, byremoving the PNA 202 from the path of the exhaust gas, the storagecapacity of the PNA 202 is maintained, and the PNA 202 can be on standbyto again adsorb NOx in the exhaust gas if needed at the next cold startcondition.

The controller 118, in addition to controlling the bypass valve 206, canalso reduce the load on the engine 102. As discussed above, during theregeneration process of the PNA 202, the controller 118 can increase theload on the engine 102 to increase the temperature of the exhaust gas.Once the regeneration process is over, the controller 118 can remove theload, such as the generator 106 or the motor 104, from the engine 102.In some instances, if the engine 102 is not needed to provide mechanicalenergy to the drive shaft 112 or electrical energy to the battery system108, the controller 118 can turn the engine off.

The controller 118, after shutting off the engine 102 may also controlthe bypass valve 206 into a position that fluidly connects the inputconduit 208 with the PNA conduit 212. In this manner, any delay inengaging the PNA 202 in the next cold start condition can be avoided.The controller 118, at the next engine startup, may still detect whetherthe cold start condition exists based on the temperature measurementsfrom the SCR catalyst temperature sensor 218 and/or the engine coolanttemperature sensor 220, and if no cold start condition exists, controlthe bypass valve 206 to direct the exhaust gas through the bypassconduit 210.

The storage capacity of the PNA 202 may deteriorate also due to exposureto exhaust gas generated from a rich air-fuel mixture. It can bebeneficial to avoid the exposure to the rich air-fuel mixture exhaustgas altogether. A rich air-fuel mixture can result from, among othercauses, a sudden high transient torque demand placed on the engine 102.For example, the user may turn a throttle (or press an acceleratorpedal) of the vehicle system 100 to indicate a desire to increase thespeed or velocity of the vehicle. This increase in speed can be viewedas an increase in the torque demand. For example, the controller 118 cantranslate the change in throttle positions or the change in acceleratorpedal positions into changes in torque demand based on the current speedand/or rpm of the engine. Based on the determined torque demand overtime, the controller 118 can determine whether the torque demand ishigh.

FIG. 6 shows plots of torque demand and lambda for an engine ininstances where only the engine is utilized to meet the high transienttorque demand. In particular, FIG. 6 shows the torque demand plot 602and a lambda plot 604 associated with an engine, such as, for example,the engine 102 shown in FIG. 1. The plots in FIG. 6 show how hightransient torque demand can result in a rich air-fuel mixture state inthe engine 102. The torque demand plot 602 shows the torque demand froma torque value of T1 to a torque value of T2 to be provided to the driveshaft 112 from time t1 to time t2. The lambda plot 604 shows the valueof lambda corresponding to the torque demand plot 602 over time when theentire torque demand is satisfied by the engine 102. That is, the torqueof the engine 102 will have to follow the torque demand plot 602. Thevalue of lambda in the lambda plot 604 refers to the air-fuelequivalence ratio, which in turn is the ratio of actual air-fuel ratioto stoichiometry for a given mixture. A value of lambda=1 indicates thatthe actual air-fuel ratio is at stoichiometry. A value of lambda >1indicates a lean air-fuel mixture, while a value of lambda <1 indicatesa rich air-fuel mixture. Rich air-fuel mixture, as mentioned above, canresult under high transient torque demands on the engine 102.

Referring again to FIG. 6, before time t1, the torque demand is at avalue T1. The controller 118 controls the engine 102 to provide thetorque demand. At this time, the engine 102 can run on a lean air-fuelmixture, which results in a value of lambda that is greater than 1. Attime t1, the torque demand begins to increase to T2. The increase intorque demand can be met by providing more fuel to the engine, which inturn can lead to a momentary increase the amount of fuel in the enginein relation to the air. As a result, the value of lambda decreases. Forhigh transient torque demands, for example, when the rate of change ofthe torque demand (e.g., (T2-T1)/(t2-t1)) exceeds a threshold value, theengine 102 may operate under transient conditions that cause the valueof lambda to decrease below 1. This indicates that the engine 102 isrunning on a rich air-fuel mixture. Such transient conditions can resultfrom the high transient torque demand, such as when the user rapidlychanges the throttle position.

Running the engine on a rich air-fuel mixture can deteriorate thestorage capacity of the PNA 202. The PNA 202 can include dispersed metalions, such as, for example, palladium ions, which form active sites foradsorption of NOx. But upon exposure to exhaust resulting from richair-fuel mixture, the metal ions form relatively larger metal particles,resulting in a reduction in the density of active sites for NOxadsorption, and thereby a reduction in the storage capacity of the PNA202. As discussed below, the controller 118 controls the operation ofthe vehicle system 100 to reduce the risk of rich air-fuel mixtureoperation during high transient torque demand.

FIG. 7 shows a flow diagram of an example process 700 for operation ofthe vehicle system under high transient torque demand. In particular,the controller 118 can execute the process 700 to reduce the risk ofrich air-fuel mixture operation of the engine 102 during high transienttorque demand, and thereby reduce the risk of reduced NOx capacity ofthe PNA 202. The process 700 includes starting the engine 102 (702). Thecontroller 118 can start the engine 102, for example, under cold startconditions, such as that discussed above in relation to FIGS. 2-5. Thatis, the controller 118 may start the engine 102 and activate the bypassvalve 206 such that the exhaust gas passes through the PNA 202. Thecontroller 118 may also start the engine 102 when no cold startconditions exist, but may still engage the PNA 202 to regenerate the PNA202. This can occur for example, if the engine was shut off by the userbefore the completion of the regeneration process of the PNA 202.

The process 700 further includes determining the presence of a hightransient torque demand (704). The controller 118 determines hightransient torque demand based on the difference between the currenttorque output of the engine and the torque demand. If the difference isgreater than a threshold value, the controller 118 determines a hightransient torque demand event. The controller 118 can determine thetorque demand based, for example, on the change in throttle position. Athrottle position sensor can provide data to the controller 118indicating the instantaneous throttle positon. The controller 118 canuse the data to determine the torque demand. For example, the controller118 can refer to a look-up-table stored in memory that can includevalues for current speed, current torque, power demand, and targettorque. The controller 118 can use the values for current speed, thecurrent torque, and the power demand to determine the target torquevalue.

The controller 118 may determine high transient torque demand based onthe rate of change in torque demand. For example, the controller 118 candetermine the presence of a high torque demand event if a ratio of thedifference between the current and the target torque to the time tochange the current torque to the target torque exceeds a thresholdvalue. Thus, if the user rapidly changes the throttle position, therapid change may cause the rate of change of torque demand to be greaterthan the threshold value, and thereby be determined as being a hightransient torque demand event. The high rate of change of torque demandcan cause the engine 102 to run rich air-fuel mixture. On the otherhand, a gradual rise in the same difference in torque demand may notcause the controller 118 to identify a high transient torque demand, asthe gradual rise in the torque demand may not cause the engine 102 torun rich air-fuel mixture, and therefore, may not generate exhaust gasthat can deteriorate the PNA 202 s. The threshold value over which thecontroller 118 can determine that the rate of change of torque demandconstitutes a high transient torque demand can be pre-determined andstored in memory. The controller 118 can then compare the determinedrate of change of torque demand to the threshold value, and if the rateof change of torque demand is greater than the threshold value,determine that a high transient torque demand condition or event hasoccurred.

The process 700 further includes, in response to determining thepresence of a high transient torque demand, utilizing the motor 104 toprovide at least a portion of the torque demand (706). The controller118 can utilize the motor 104 to provide at least a portion of thetorque demand to mitigate the risk of rich air-fuel mixture operation ofthe engine 102. FIG. 8 shows example torque and lambda plots for thevehicle when operating in a power split mode to reduce the risk of richair-fuel mixture operation. In particular, FIG. 8 shows a torque demandplot 802, which is similar to the torque demand plot 602 shown in FIG.6, a motor torque plot 804, which depicts the torque provided by themotor 104, an engine torque plot 806, which depicts the torque providedby the engine 102, a first lambda plot 808, which is similar to thelambda plot 604 shown in FIG. 6, and a second lambda plot 810corresponding to the engine torque plot 806. In contrast with FIG. 6,where the controller 118 utilizes only the engine 102 to meet the torquedemand, in FIG. 8, the controller 118 utilizes the motor 104 to meet aportion of the torque demand. For example, the controller 118, at timet1, increases the torque output of the motor 104 so that the torquedemand from time t1 onwards is met by the motor 104. In particular, thecontroller 118 can increase the electrical power provided to the motor104 through the battery system 108 to increase the torque output of themotor 104. Additionally, the controller 118 can control the drivetrain116 to couple the motor 104 with the drive shaft 112 so that torquegenerated by the motor 104 is transferred to the drive shaft 112.

The process 700 includes determining whether the motor torque meets thetorque demand (716). In particular, the controller 118 can determinewhether the increase in the motor torque (Tm) has met the torque demand(Tdemand). If the increase in motor torque has met the torque demand,then the controller 118 may continue using the motor 104 or the engine102 to provide the torque to the drive shaft 112 (714). The controller118, having determined that the torque demand has been met by the motor104, can identify that the high transient torque demand has subsided,and that the vehicle 100 is not in a high transient torque demand state.In some implementations, the controller 118 may switch back to utilizingonly the engine 102 to provide the torque to the drive shaft 112. Asthere is not high transient torque demand, using only the engine 102 toprovide torque may not cause the engine 102 to operate in a richair-fuel mixture state. If the controller 118 determines that theprevious increase in the motor 104 torque has met the torque demand,controller 118 can determine whether the torque of the motor 104 hasreached a predetermined torque value.

The process 700 includes utilizing the motor 104 provide the torquedemand until the torque of the motor 104 reaches a predetermined torquevalue (708). In particular, the controller 118 can continue to transferpower from the motor 104 to the drive shaft 112 until the motor torque(Tm) reaches a predetermined torque value (Tp). In some examples, thepredetermined torque value can be the maximum torque capacity of themotor 104. The predetermined value may be a percentage (e.g., 50%-90%)of the maximum torque capacity of the motor 104. Once the controller 118determines that the motor torque has reached the predetermined torquevalue, the motor can engage the engine 102 to meet the torque demand.Referring to FIG. 8, as shown in the motor torque plot 804, the motortorque increases to meet the torque demand from time t1 to time t3, atwhich time the controller 118 determines that the motor torque hasreached the predetermined torque value.

In response to the motor torque reaching the predetermined torque value,but the torque demand not having been met, the engine 102 is utilized tomeet the remainder of the torque demand (710). The controller 118, upondetermining that the motor torque has reached the predetermined value,can utilize the engine 102 to meet the remainder of the torque demand.For example, the controller 118, at time t3, can control the drivetrain116 to couple the engine 102 with the drive shaft 112, thereby providingthe engine torque to the drive shaft 112.

The process 700 includes continuing to provide the torque from theengine 102 to the drive shaft until the high transient torque demand ismet (712). Referring to FIG. 8, the controller 118 can continue toutilize the engine 102 to meet the torque demand until time t3. Theincrease in the engine torque from time t2 to t3 results in a decreasein the value of lambda, as shown in the second lambda plot 810 in FIG.8. The decrease in the value of lambda can be a result of increased fuelsupplied to the engine, and the resulting decrease in the air-fuelmixture ratio. However, as the magnitude of the torque demand met by theengine 102 is relatively less than that when the engine 102 alone isutilized to meet the entire torque demand, the value of lambda does notdecrease below the value of 1. Therefore, the engine 102 runs on arelatively lean air-fuel mixture throughout the duration that itprovides torque to meet the high transient torque demand. As a result,the risk of the exhaust gas produced by the engine 102 deteriorating thestorage capacity of the PNA 202 is also reduced. By utilizing the motor104 to meet at least a portion of the high transient torque demand, therisk of the value of lambda falling below 1 is reduced. After the hightransient torque demand is met, the controller 118 can continue toutilize the engine 102, the motor 104, or both, to provide power to thedrive shaft 112 (at 714).

In the example discussed above in relation to FIGS. 7-8, the motor 104is utilized first to provide torque in response to high transient torquedemand. In some other examples, the motor 104 can be utilized anywhereduring the period of high transient torque demand. For example, thecontroller 118 can utilize the engine 102 to meet the high transienttorque demand first for a duration, and engage the motor 104 to providethe remainder of the high transient torque demand after the duration.The controller 118 can select the duration to be short enough to ensurethat the value of lambda does not decrease below the value of 1. Thecontroller 118 may also alternate between the engine 102 and the motor104 throughout the duration of the high transient torque demand toensure that the value of lambda does not decrease below the value of 1.

It should be understood that the solutions discussed above are notlimited to vehicle systems, and can be applied to any system thatincludes an engine and an aftertreatment system or additionally includesa motor-generator.

For the purpose of this disclosure, the term “coupled” means the joiningof two members directly or indirectly to one another. Such joining maybe stationary or moveable in nature. Such joining may be achieved withthe two members or the two members and any additional intermediatemembers being integrally formed as a single unitary body with oneanother or with the two members or the two members and any additionalintermediate members being attached to one another. Such joining may bepermanent in nature or may be removable or releasable in nature.

It should be noted that the orientation of various elements may differaccording to other exemplary embodiments, and that such variations areintended to be encompassed by the present disclosure. It is recognizedthat features of the disclosed embodiments can be incorporated intoother disclosed embodiments.

It is important to note that the constructions and arrangements ofapparatuses or the components thereof as shown in the various exemplaryembodiments are illustrative only. Although only a few embodiments havebeen described in detail in this disclosure, those skilled in the artwho review this disclosure will readily appreciate that manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter disclosed. For example,elements shown as integrally formed may be constructed of multiple partsor elements, the position of elements may be reversed or otherwisevaried, and the nature or number of discrete elements or positions maybe altered or varied. The order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes and omissionsmay also be made in the design, operating conditions and arrangement ofthe various exemplary embodiments without departing from the scope ofthe present disclosure.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other mechanisms and/or structures for performing thefunction and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the inventiveembodiments described herein. More generally, those skilled in the artwill readily appreciate that, unless otherwise noted, any parameters,dimensions, materials, and configurations described herein are meant tobe exemplary and that the actual parameters, dimensions, materials,and/or configurations will depend upon the specific application orapplications for which the inventive teachings is/are used. Thoseskilled in the art will recognize, or be able to ascertain using no morethan routine experimentation, many equivalents to the specific inventiveembodiments described herein. It is, therefore, to be understood thatthe foregoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed and claimed. Inventive embodiments of the present disclosureare directed to each individual feature, system, article, material, kit,and/or method described herein. In addition, any combination of two ormore such features, systems, articles, materials, kits, and/or methods,if such features, systems, articles, materials, kits, and/or methods arenot mutually inconsistent, is included within the inventive scope of thepresent disclosure.

Also, the technology described herein may be embodied as a method, ofwhich at least one example has been provided. The acts performed as partof the method may be ordered in any suitable way unless otherwisespecifically noted. Accordingly, embodiments may be constructed in whichacts are performed in an order different than illustrated, which mayinclude performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

The claims should not be read as limited to the described order orelements unless stated to that effect. It should be understood thatvarious changes in form and detail may be made by one of ordinary skillin the art without departing from the spirit and scope of the appendedclaims. All embodiments that come within the spirit and scope of thefollowing claims and equivalents thereto are claimed.

1. A system, comprising: a catalyst for receiving and treating exhaustgas generated by an engine; a passive NOx adsorber (PNA) positionedupstream of the catalyst and fluidly coupled with the catalyst; a bypassvalve positioned upstream of the catalyst and the PNA; and a controller,configured to: determine that an effectiveness of the catalyst inreducing NOx is below a threshold level, in response to determining thatthe effectiveness of the catalyst in reducing NOx is below the thresholdlevel, control the bypass valve to direct exhaust gas to the PNA,subsequent to controlling the bypass valve to direct exhaust gas to thePNA, determining that the effectiveness of the catalyst is no longerbelow the threshold level, and in response to determining that theeffectiveness of the catalyst is no longer below the threshold level:continue to control the bypass valve to direct exhaust gas to the PNA;and after PNA regeneration conditions are met, control the bypass valveto direct exhaust gas to the catalyst bypassing the PNA.
 2. The systemof claim 1, wherein the controller is further configured to, whilecontinuing to control the bypass valve to direct exhaust gas to the PNA,increase a load on the engine.
 3. The system of claim 2, wherein theload on the engine is increased by mechanically engaging a generatorwith the engine, and utilizes resulting electrical energy generated bythe generator to charge a battery system.
 4. The system of claim 1,wherein the effectiveness of the catalyst is determined to be below thethreshold level based on a determination that a temperature of thecatalyst is below a first threshold value.
 5. The system of claim 1,wherein the effectiveness of the catalyst is determined to be below thethreshold level based on a determination that an NOx at the output ofthe catalyst is below a first threshold value.
 6. The system of claim 1,wherein the PNA regeneration conditions are met upon an elapse of apredetermined duration for which exhaust gas is directed through the PNAafter determining that the effectiveness of the catalyst is no longerbelow the threshold level.
 7. The system of claim 1, wherein thecontroller is further configured to adjust the bypass valve such thatexhaust gas received at an input port is proportionally output between aPNA conduit and a bypass conduit.
 8. The system of claim 1, wherein thecontroller is further configured to adjust the bypass valve such thatthe exhaust gas received at an input port is output only to a PNAconduit and not a bypass circuit.
 9. The system of claim 1, wherein thePNA comprises NOx adsorbing elements including at least one ofpalladium, platinum, sliver, zeolite, Al2O3, and CeO2-containingmaterials.
 10. A system comprising: a catalyst for receiving andtreating exhaust gas generated by an engine; a passive NOx adsorber(PNA) positioned upstream of the catalyst and fluidly coupled with thecatalyst; a bypass valve positioned upstream of the catalyst and thePNA; and a controller configured to: while controlling the bypass valveto direct exhaust gas to the PNA, detect a torque demand that is greaterthan a threshold value; responsive to detecting that the torque demandis greater than the threshold value, engage a motor, coupled with abattery system, with a drive shaft of the system to meet at least aportion of the torque demand; and in response to the engagement of themotor with the drive shaft not meeting all of the torque demand, engagethe engine with the drive shaft to meet a remainder of the torquedemand.
 11. The system of claim 10, wherein the engine is engaged withthe drive shaft in response to determining that the motor has reachedits maximum torque capacity, but that all of the torque demand has notbeen met.
 12. The system of claim 10, wherein the controller is furtherconfigured to translate a change in throttle positions into changes intorque demand based on a current speed and/or rpm of the engine, andwherein a throttle position sensor provides data to the controllerindicating the throttle positon.
 13. The system of claim 10, wherein thecontroller is further configured to determine the presence of a hightorque demand event in response to a ratio of a difference between acurrent torque and a target torque to a time to change the currenttorque to the target torque exceeding a threshold value.
 14. A methodfor operating passive nitrogen oxide absorbers (PNA) in an exhaustaftertreatment system, the method comprising: determining that aneffectiveness of a catalyst in reducing NOx is below a threshold level,the catalyst for receiving and treating exhaust gas generated by anengine; in response to determining that the effectiveness of thecatalyst in reducing NOx is below the threshold level, controlling abypass valve to direct exhaust gas to the PNA, the PNA positionedupstream of the catalyst and fluidly coupled with the catalyst and thebypass valve positioned upstream of the catalyst and the PNA; subsequentto controlling the bypass valve to direct exhaust gas to the PNA,determining that the effectiveness of the catalyst is no longer belowthe threshold level; and in response to determining that theeffectiveness of the catalyst is no longer below the threshold level:continuing to control the bypass valve to direct exhaust gas to the PNA;and after PNA regeneration conditions are met, controlling the bypassvalve to direct exhaust gas to the catalyst bypassing the PNA.
 15. Themethod of claim 14, wherein the method further comprises, whilecontinuing to control the bypass valve to direct exhaust gas to the PNA,increasing a load on the engine.
 16. The method of claim 15, wherein theload on the engine is increased by mechanically engaging a generatorwith the engine, and further comprising utilizing resulting electricalenergy generated by the generator to charge a battery system.
 17. Themethod of claim 14, wherein the effectiveness of the catalyst isdetermined to be below the threshold level based on a determination thata temperature of the catalyst is below a first threshold value.
 18. Themethod of claim 14, wherein the PNA regeneration conditions are met uponan elapse of a predetermined duration for which exhaust gas is directedthrough the PNA after determining that the effectiveness of the catalystis no longer below the threshold level.
 19. The method of claim 14,further comprising: while controlling the bypass valve to direct exhaustgas to the PNA, detecting a torque demand that is greater than athreshold value; responsive to detecting that the torque demand isgreater than the threshold value, engaging a motor, coupled with abattery system, with a drive shaft of the system to meet at least aportion of the torque demand; and in response to the engagement of themotor with the drive shaft not meeting all of the torque demand,engaging the engine with the drive shaft to meet a remainder of thetorque demand.
 20. The method of claim 19, wherein the engine is engagedwith the drive shaft in response to determining that the motor hasreached its maximum torque capacity, but that all of the torque demandhas not been met.